Waste water entering a waste water treatment facility normally includes a substantial quantity of sand, grit and plastic, in addition to suspended organic matter. One of the first steps in treating the waste water is to remove these non-organic solids so that the waste water can then be treated by oxidation and biodegradation techniques without the non-organic solids fouling or wearing out the mechanical components of the facility. Frequently, the plant design requires that inorganic solids having a specific gravity of 2.65 and higher and a size of 65 mesh or larger should be removed before further treatment of the waste water.
Although it is important that the non-organic solids be removed early in the treatment process, it is equally important that the organic solids such as sewage sludge not be removed at that point. This is because, at that point, the waste water has not been subjected to the oxidation and biodegradation process that clean the water by consuming the sewage. Therefore, if the organic solids are removed along with the inorganic solids before the treatment process, then the removed solids will contain raw sewage which presents difficult problems with the storage and disposal of a possibly hazardous waste.
Fortunately, the inorganic solids are generally distinguishable from the organic solids on the basis of their specific gravities. Inorganic solids such as sand and grit tend to be much denser than organic solids such as sludge. Therefore, inorganic solids tend to settle to the bottom of the fluid stream quicker than organic solids.
In most of the existing waste water treatment facilities, it is this difference in density between inorganic solids and organic solids that it is relied upon to remove the inorganic solids. Typically, the incoming waste water flows through an inorganic removal chamber that may be referred to as "grit chamber". The grit chamber normally operates with a continuous flow through it rather than as a batch at a time. Waste water flows into the chamber through an inlet and out of the chamber through an outlet. At the bottom of the chamber is generally a collection space.
Existing grit chambers include several designs by Weis. In U.S. Pat. No. 3,941,698 by Weis, the grit chamber includes an upper settling chamber and a lower grit storage chamber, with a transition surface dividing the two. Rotating paddles within the settling chamber positioned above the transition surface, cause liquid entering the settling chamber at the outer periphery to rotate about a vertical axis within the chamber to produce a forced vortex. The forced vortex results in an upward spiral flow, so that the flow enters at the settling chamber periphery through an inlet, and flows spirally upward and exits the settling chamber from an outlet, also at the periphery. Dense particles fall through an opening in the transition surface between the settling chamber and the storage chamber coinciding with the vertical axis of the forced vortex.
Variations of the device disclosed in U.S. Pat. No. 3,941,698, discussed above, include those disclosed in U.S. Pat. Nos. 4,107,038 and 4,767,532. Such patents disclose modifications in the settling chamber configuration to enhance the liquid flow. Other devices in the prior art include those disclosed in U.S. Pat. No. 4,519,907 by Rooney.
It has now been discovered that further improvement is possible in the grit chamber efficiency by further modifying the settling chamber configuration. This is important, because the prior art devices result in fairly high turbulence, which interferes with effective and reliable settling and also consumes additional energy. It is also desirable to employ a design which facilitates the removal of finer grits.